The present invention relates to a PCM communication apparatus for multiplexing signals on a plurality of channels and, more particularly, to a PCM communication apparatus constructed such that a code allocated to a time slot of a non-connected channel when the number of channels connected to a multiplexer unit is smaller than allowed multiplicity and a code appearing in a time slot of a particular channel which is connected to the multiplexer unit but not supplied with an input signal, i.e., idle channel, are identical with each other.
In a PCM communication apparatus of the type described, it has been customary to encode input signals channel by channel and cause a multiplexer unit to time division multiplex output signals on the respective channels for sending them out to a communication link.
Electronic circuits assigned to the channels are respectively accommodated in packages to constitute channel units which are in turn connected to input connectors of the multiplexer unit. Data lines at the input side of the multiplexer unit are pulled up or pulled down so that the input levels of the channel signals may be freed from instability. Usually, the input data lines are pulled up because should they be pulled down, the level of logical ONEs which are included in channel codes would be lowered proportionally to division of resistance effected by pull-down resistors and, thereby, liable to be erroneously determined as ZEROs. In the pull-up type arrangement, when any of the channel units is not connected to the multiplexer unit, a disengaged channel input level at the multiplexer unit becomes a ONE so that the multiplexer unit multiplexes a sequence of ONEs representative of a disconnected state together with data on the other channels. However, where a code consisting of a sequence of ONEs is one of the channels, the non-connected channel appears as if it were busy.
To settle such a problematic situation, there has been proposed a system in which a multiplexer unit uses special monitor lines for monitoring connections of channels. In this system, when any channel is not connected, the multiplexer unit detects an open state of the associated monitor line at its disengaged channel input side. In response to the detection of the open state, the multiplexer unit generates a code identical with a particular one which is generated when any channel is idle, that code being multiplexed together with other channel data. Hence, the non-connected state of the channel unit is treated as an idle state of channel data. The problem with this implementation is that the monitor lines must be connected one to each channel unit for the detection of disengaged channels which increases the number of connectors necessary for the connection of the monitor lines, resulting in an expensive construction and a disproportionate space requirement.